Implement `Extend` for `World`

It may be possible to generically implement `Extend` for `World` in a generic way that allows for extending from any iterator that yields values implementing `Entity`. If this is found to be possible, `IntoIterator` could also be implemented for `Batch` (which still needs to be renamed, by the way), allowing `extend` to work with that `struct`.

One main issue is that our current `extend` method returns `EntityIdentifier`s corresponding to the inserted values. This would not be possible with the `Extend` trait. At the very least, however, `Batch` should be made to implement `IntoIterator`, and also should work with inputs besides `Vec`.